.\" Copyright (c) 1983 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.\"	@(#)ec.4	6.4 (Berkeley) 8/20/87
.\"
.TH EC 4 "August 20, 1987"
.UC 2
.SH NAME
ec \- 3Com 10 Mb/s Ethernet interface
.SH SYNOPSIS
.ft B
.nf
/sys/conf/SYSTEM:
	NEC	\fIec_controllers\fP	# 3Com Ethernet
.fi
.ft R
.SH DESCRIPTION
The
.I ec
interface provides access to a 10 Mb/s Ethernet network through
a 3com controller.
.PP
The hardware has 32 kilobytes of dual-ported memory on the UNIBUS. 
This memory
is used for internal buffering by the board, and the interface code reads
the buffer contents directly through the UNIBUS.
The address of this memory is given in the \fIflags\fP field
in the configuration file.
The first interface normally has its memory at Unibus address 0.
.PP
Each of the host's network addresses
is specified at boot time with an SIOCSIFADDR
ioctl.  The
.I ec
interface employs the address resolution protocol described in
.IR arp (4P)
to dynamically map between Internet and Ethernet addresses on the local
network.
.PP
The interface normally tries to use a ``trailer'' encapsulation
to minimize copying data on input and output.
The use of trailers is negotiated with ARP.
This negotiation may be disabled, on a per-interface basis,
by setting the IFF_NOTRAILERS
flag with an SIOCSIFFLAGS ioctl.
.PP
The interface software implements an exponential backoff algorithm
when notified of a collision on the cable.  This algorithm utilizes
a 16-bit mask and the VAX-11's interval timer in calculating a series
of random backoff values.  The algorithm is as follows:
.TP 5
1.
Initialize the mask to be all 1's.
.TP 5
2.
If the mask is zero, 16 retries have been made and we give
up.
.TP 5
3.
Shift the mask left one bit and formulate a backoff by
masking the interval timer with the smaller of the complement of this mask
and a 5-bit mask, resulting in a pseudo-random number between 0 and 31.
This produces the number of slot times to delay,
where a slot is 51 microseconds.
.TP 5
4.
Use the value calculated in step 3 to delay before retransmitting
the packet.
The delay is done in a software busy loop.
.SH DIAGNOSTICS
.BR "ec%d: send error" .
After 16 retransmissions using the
exponential backoff algorithm described above, the packet
was dropped.
.PP
.BR "ec%d: input error (offset=%d)" .
The hardware indicated an error
in reading a packet off the cable or an illegally sized packet.
The buffer offset value is printed for debugging purposes.
.PP
.BR "ec%d: can't handle af%d" .
The interface was handed
a message with addresses formatted in an unsuitable address
family; the packet was dropped.
.SH SEE ALSO
intro(4N), inet(4F), arp(4P)
.SH BUGS
The hardware is not capable of talking to itself.  The software
implements local sending and broadcast by sending such packets to the
loop interface.  This is a kludge.
.PP
Backoff delays are done in a software busy loop.  This can degrade the
system if the network experiences frequent collisions.
